Libguestfs - Jul 2014 - Re: virt-resize: support to MBR logical partitions and some question

Hi,

I'm adding support to resizing logical partitions(patch is in progess).
But encounter an error when adding a logical partition in dest image:

  virt-resize: libguestfs error: part_add: parted: /dev/sdb: Warning: The
resulting partition is not properly aligned for best performance.
  Error: Error informing the kernel about modifications to partition /dev/sdb5
-- Device or resource busy.  This means Linux won't know about any changes
you made to /dev/sdb5 until you reboot -- so you shouldn't mount it or use
it in any way before rebooting.
  Error: Failed to add partition 5 (Device or resource busy)

The error is actually no harm since the logical partition has been added
successfully, and I don't want to inform kernel at all. But it prevents
virt-resize from adding further logical partitions.

I can ignore the error when adding logical partitions manually using parted.
The question is, is there any way to ignore such errors in virt-resize?

Regards,
Hu Tao

On Wed, Jun 04, 2014 at 10:21:41AM +0800, Hu Tao wrote:
> Hi,
> 
> I'm adding support to resizing logical partitions(patch is in progess).
> But encounter an error when adding a logical partition in dest image:
> 
>   virt-resize: libguestfs error: part_add: parted: /dev/sdb: Warning: The
resulting partition is not properly aligned for best performance.
>   Error: Error informing the kernel about modifications to partition
/dev/sdb5 -- Device or resource busy.  This means Linux won't know about any
changes you made to /dev/sdb5 until you reboot -- so you shouldn't mount it
or use it in any way before rebooting.
>   Error: Failed to add partition 5 (Device or resource busy)
> 
> The error is actually no harm since the logical partition has been added
> successfully, and I don't want to inform kernel at all. But it prevents
> virt-resize from adding further logical partitions.
> 
> I can ignore the error when adding logical partitions manually using
parted.
> The question is, is there any way to ignore such errors in virt-resize?
This may indicate a bug in the daemon.

Normally after parted runs, it tries to get the kernel to re-read
partition tables.  This can fail because for several reasons -- for
example, because a partition is mounted (and for many other reasons,
including udev and/or the kernel just getting confused.

We try to avoid this if we can by various tricks, eg. see use of
udev_settle function in the daemon.

Can you come up with a minimal reproducer (eg. using just guestfish
commands)?  If so, then file a bug.
>From virt-resize code you can catch and ignore exceptions, but this is
potentially dangerous since it doesn't distinguish between this
"harmless" error and other actual errors.

I'm on holiday at the moment, but I'll try to offer help and advice on
modifying virt-resize when I can.  I'm back next week.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
Fedora Windows cross-compiler. Compile Windows programs, test, and
build Windows installers. Over 100 libraries supported.
http://fedoraproject.org/wiki/MinGW

On Wed, Jun 04, 2014 at 10:02:55AM +0100, Richard W.M. Jones
wrote:
> 
> On Wed, Jun 04, 2014 at 10:21:41AM +0800, Hu Tao wrote:
> > Hi,
> > 
> > I'm adding support to resizing logical partitions(patch is in
progess).
> > But encounter an error when adding a logical partition in dest image:
> > 
> >   virt-resize: libguestfs error: part_add: parted: /dev/sdb: Warning:
The resulting partition is not properly aligned for best performance.
> >   Error: Error informing the kernel about modifications to partition
/dev/sdb5 -- Device or resource busy.  This means Linux won't know about any
changes you made to /dev/sdb5 until you reboot -- so you shouldn't mount it
or use it in any way before rebooting.
> >   Error: Failed to add partition 5 (Device or resource busy)
> > 
> > The error is actually no harm since the logical partition has been
added
> > successfully, and I don't want to inform kernel at all. But it
prevents
> > virt-resize from adding further logical partitions.
> > 
> > I can ignore the error when adding logical partitions manually using
parted.
> > The question is, is there any way to ignore such errors in
virt-resize?
> 
> This may indicate a bug in the daemon.
> 
> Normally after parted runs, it tries to get the kernel to re-read
> partition tables.  This can fail because for several reasons -- for
> example, because a partition is mounted (and for many other reasons,
> including udev and/or the kernel just getting confused.
> 
> We try to avoid this if we can by various tricks, eg. see use of
> udev_settle function in the daemon.
It seems udev_settle() doesn't help in the case. I think I need to digg
more of the code for a sulation.
> 
> Can you come up with a minimal reproducer (eg. using just guestfish
> commands)?  If so, then file a bug.
No, because guestfish doens't support adding logical partition but the
problem is reproduced with logical partition only.
> 
> >From virt-resize code you can catch and ignore exceptions, but this is
> potentially dangerous since it doesn't distinguish between this
> "harmless" error and other actual errors.
> 
> I'm on holiday at the moment, but I'll try to offer help and advice
on
> modifying virt-resize when I can.  I'm back next week.
Cool!

Hu

Hi Rich,

On Wed, Jun 04, 2014 at 10:02:55AM +0100, Richard W.M. Jones
wrote:
> 
> On Wed, Jun 04, 2014 at 10:21:41AM +0800, Hu Tao wrote:
> > Hi,
> > 
> > I'm adding support to resizing logical partitions(patch is in
progess).
> > But encounter an error when adding a logical partition in dest image:
> > 
> >   virt-resize: libguestfs error: part_add: parted: /dev/sdb: Warning:
The resulting partition is not properly aligned for best performance.
> >   Error: Error informing the kernel about modifications to partition
/dev/sdb5 -- Device or resource busy.  This means Linux won't know about any
changes you made to /dev/sdb5 until you reboot -- so you shouldn't mount it
or use it in any way before rebooting.
> >   Error: Failed to add partition 5 (Device or resource busy)
> > 
> > The error is actually no harm since the logical partition has been
added
> > successfully, and I don't want to inform kernel at all. But it
prevents
> > virt-resize from adding further logical partitions.
> > 
> > I can ignore the error when adding logical partitions manually using
parted.
> > The question is, is there any way to ignore such errors in
virt-resize?
> 
> This may indicate a bug in the daemon.
> 
> Normally after parted runs, it tries to get the kernel to re-read
> partition tables.  This can fail because for several reasons -- for
> example, because a partition is mounted (and for many other reasons,
> including udev and/or the kernel just getting confused.
> 
> We try to avoid this if we can by various tricks, eg. see use of
> udev_settle function in the daemon.
> 
> Can you come up with a minimal reproducer (eg. using just guestfish
> commands)?  If so, then file a bug.
guestfish just works with my patch(attached):

---
[root@f20-x64 libguestfs-git]# rm -f delme.img && truncate -s 5G
delme.img && ./run guestfish -a delme.img <<EOF
run
part-init /dev/sda msdos
part-add /dev/sda extended 512 -1
part-add /dev/sda logical 2049 -1
EOF

[root@f20-x64 libguestfs-git]#  fish/guestfish  -a delme.img <<EOF
run
list-partitions
EOF                              
/dev/sda1
/dev/sda5
---


But virt-resize doesn't. The error message is:

Setting up initial partition table on test.img ...
virt-resize: libguestfs error: part_add: parted: /dev/sdb: Warning: The
resulting partition is not properly aligned for best performance.
Error: Error informing the kernel about modifications to partition /dev/sdb5 --
Device or resource busy.  This means Linux won't know about any changes you
made to /dev/sdb5 until you reboot -- so you shouldn't mount it or use it in
any way before rebooting.
Error: Failed to add partition 5 (Device or resource busy)

This is the same as I reported earlier. At this time /dev/sdb has been added
successfully despite of the error message. But the bad is I can't write
to /dev/sdb5, aka copy /dev/sda5 to /dev/sdb5. The error message is:

---
Setting up initial partition table on test.img ...
Copying /dev/sda1 ...
 100%
⟦▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒⟧
00:00
Copying /dev/sda2 ... 
 100%
⟦▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒⟧
00:00
Copying /dev/sda3 ...
Copying /dev/sda5 ... 
virt-resize: libguestfs error: copy_device_to_device:
copy_device_to_device_stub: /dev/sdb5: No such file or directory
---

The solution could be:

1. as the error message suggests, rebooting. But I don't known how since it
   runs in an applicance if I'm right.

2. fix parted informing the kernel about modifications to partitions. Do
   you have any insight of this? If you think this is a possible
   solution I'll start invetigate it.


Regards,
Hu